Asymmetric Wetting Hysteresis on Hydrophobic Microstructured Surfaces

The wetting behavior of hydrophobic, microstructured surfaces containing arrays of pillars or holes has been investigated. The size of the surface features was fixed (20/mu m), while their separation wits varied to adjust the area fraction (0-80%). The wettability of structured surfaces for liquids resting in the Cassie state is strongly dependent on the continuity of the solid component. Microstructured square pillars and holes showed distinct, asymmetric wetting hysteresis, consistent with our previous observations oil flat, chemically heterogeneous surfaces. Furthermore, clear trends for the magnitude of contact angle hysteresis versus area fraction for the two types of microstructured Surfaces are evident. The pinning energy associated with these surface features is estimated.